The invention relates to a method for coating a roll of a paper machine with a powder comprising a thermoplastic and/or specialty plastic and a roll manufactured by the method. Coated rolls are used for very different purposes and applications in paper machines during the paper-making process and in post-handling machines for paper. Among the applications for such coated rolls are, e.g., press rolls, suction rolls, soft rolls in calenders and super calenders and the like. Different quality requirements are desired for coating the roll in different applications and in different processes. Some conventional quality factors for the roll coating are its hardness at a given temperature, temperature resistance, press resistance, chemical resistance, surface smoothness, resistance against mechanical damages, elasticity, surface energy, loosing properties of the paper, conductivity, and non-ageing.
Conventionally rolls of paper machines have been coated with rubber, polyurethane or epoxy. These polymeric materials are especially suitable for coating large rolls of manufacturing for technical reasons. One- or two-component polyurethane and epoxy compounds are available ill fluid form in which case these compounds can be cast in a form or rotation casting. It is also very easy to mix these polymeric materials with different fillers and additives to obtain new properties for the coating material. In addition to the form and rotation casting, suitable manufacturing techniques (coating techniques) for the polyurethane and epoxy include extrusion, spraying, filament winding, tape winding, spun casting and different impregnated mats.
Epoxy (a thermo-setting plastic) and polyurethane (a thermo-setting plastic or an elastomer) are materials which are used as roll coatings because, in addition to manufacturing and technical advantages, such polymer materials have advantageous properties. For example, polyurethane has good dynamic and abrasion properties and epoxy has been providing corrosion properties. The properties of the epoxy are retained also at higher operating temperatures.
The use of thermo-setting plastics as roll coatings has mainly been restricted by the loss of the advantageous properties with increasing coating temperatures and by manufacturing problems (expressly with respect to the coating of large rolls). However, a significant development has occurred during the last 10 years with respect to thermo-setting plastics.
In FIG. 1, a classification of actual thermo-setting plastics is presented generally.
In the following Table 1, there is a list according to ISO 1043-1 of abbreviations and names for some polymers. It also includes possible homopolymers.
TABLE 1 ______________________________________ CA Cellulose-acetate CAB Cellulose acetate butyrate CN Cellulose nitrate CP Cellulose propionate EP Epoxy or epoxide MF Melamine formaldehyde
Polyamide (quality is expressed with numbers)
Polyamide-imide PAN Polyacrylnitrile PB Polybutene-1 PBT Polybutene terephtalate PC Polycarbonate PCTFE Polychlorotrifluorethene PDAP Polydiallyl phthalate PE Polyethene PEI Polyether-imide PEK Polyetherketone PEEK + derivative Polyetheretherketone PES Polyethersulfon PET Polyethenterephtalate PF Phenol formaldehyde PFA Perfluoroalcoxyalkane PI Poly-imide PIB Polyisobutene PMI Polymetakryl-imide PMMA Polymethylmethacrylate PMP Poly-4-methylpentene-1 POM Polyoxyrnethene or polyacetal PP Polypropene PPE Polyphenylenether, earlier polyphenylen oxide PPO PPS Polyphenylen sulfide PS Polystyrene PSU Polysulfone PTEE Polytetrafluoroethene PUR Polyurethane PVC Polyvinyl chloride PVDC Polyvinyliden chloride PVDF Polyvinyliden fluoride PVF Polyvinylfluoride SI Silicon UF Ureaformaldehyde UP Unsaturated polyester ______________________________________
The group of specialty plastics are especially interesting. Typical properties for plastics belonging to this group have good temperature resistances (260.degree. C.), good mechanical properties (which properties are retained even in high temperatures in spite of high tensile strengths and good hardness properties), retained elasticity and a low impregnation of water.
In Table 2, properties of the specialty plastic PEEKK are presented as a function of the temperature.
TABLE 2 __________________________________________________________________________ Temperature property -40.degree. C. 23.degree. C. 80.degree. C. 120.degree. C. 150.degree. C. 220.degree. C. Unit __________________________________________________________________________ Tensile 129 108 76 56 49 -- N/mm.sup.2 strength Ultimate 4 6 6,5 9 10 -- % elongation Tear strength 109 86 69 55 48 35 N/mm.sup.2 Tear elonga- 30 28 100 124 128 142 % tion Tensile-E- 4150 4000 3490 3340 3100 230 N/mm.sup.2 Modulus Bending 131 120 107 91 84 8 N/mm.sup.2 stress Bending-E- 3860 3640 3370 3120 3010 240 N/mm.sup.2 Modulus Notch impact 9 9 mJ/mm.sup.2 toughness (Charpy) __________________________________________________________________________
The advantageous properties of the specialty plastics at high temperatures are based on the principle of the substitution of the conventional aliphatic bond with an aromatic bond. The specialty plastics afford properties which are suitable for roll coatings, for example, in paper machines, carton machines and paper refineries. They can be used either as reinforced plastics or not. However, the specialty plastics are thermo-plastics and their processing methods are typical for thermo-plastics. Specialty plastics are available in granulates from which such fabricates as films, discs, tubes and bars are manufactured by such processes as injection moulding and extrusion.
Thermo-plastics are also available in powder form, in which case possible manufacturing techniques include dispersion spraying, electrostatic powder spraying, fluidized bed coating, flame spraying, plasma spraying and rotormolding. Filament winding and tape winding are conventionally suitable manufacturing techniques for thermo-setting plastics. However, recently the use of these two techniques has been more common also for thermo-setting plastics. Thermo-setting plastics and also specialty plastics can thus be obtained in powder form.
Large rolls can be coated with plastic powder by processes such as the following:
1. Electrostatic spraying--This process is usually used for relatively thin coatings. The porosity of the coatings formed by this process is high. In this case, for specialty plastics, the pre-heating and post-heating temperatures of the roll body are high which is not advantageous with respect to the paper machine rolls (carton and paper ref.). PA1 2. Fluidized bed coating--This process is, as in the case of the electrostatic spraying, usually used only for thin coatings having a high porosity. The pre-heating/post-heating temperatures of the roll bodies are high. Manufacturing problems are associated with this method. PA1 3. Dispersion spraying--In this process, the plastic powder is in the form of a dispersion in some suitable solvent. The dispersion is sprayed onto a surface of a body. The solvent evaporates/is evaporated away such that a very thin coating film is left on the surface of the working piece which often requires further temperature treating. Another possibility is to mix the plastic powder among some one- or two -component polymer. When the one- or two- component polymer reacts, a matrix is formed in which the plastic powder is left. PA1 4. Rotormoulding technique--This process is used to coat interior surfaces and therefore cannot be used for coating outer surfaces of rolls. PA1 5. Flame spraying--This process presents problems as described in the following.
Only standard plastics (for example PE, EVA, PP) can, to some extent, be sprayed without pre-heating the working piece which may be a roll. However, these plastics are not suitable for roll coatings with precise technical requirements.
With respect to the process of flame spraying with a specialty plastic, in such a process the working piece must be heated to a temperature as high as possible when thick coatings are desired. However, the temperature can not exceed a given threshold at which the plastic will burn. Also, the roll construction can set a limit for the temperature above which the roll construction will be adversely affected. Working pieces with thin walls need a higher pre-heating temperature than compact pieces. It is especially difficult to flame spray pieces of different thicknesses.
In this process of flame spraying, the plastic coating is sprayed in layers. The effect of the pre-heating decreases considerably after the first spraying layer. The piece is cooled down as the temperature of the roll is not kept constant. Even if the temperature would be kept constant, the coating being formed would become an isolate when it thickens. Because of the differences in the cooling rates, the temperature differences are increased. Thus, the first plastic layer isolates the heat coming from the working piece which consequently limits the coating thickness.
In a situation where the coating is too thick, and in a plastic coating which lacks heat energy in the outer layer, the melt drops of the plastic separate. The construction of the roll coating thereby becomes worse, i.e. the inner strength is weak and the crystallization degree incorrect.
Similar difficulties also appear in connection with conventional plasma spraying. In conventional plasma spraying, the heat effect of the spraying is formed so that the electric energy forms an arc between a wolfram cathode and an annular copper anode. A gas, or gas mixture, is led to the arc which is strongly heated up so that the gas molecules disintegrate to atoms and the atoms further disintegrate to ions and electrons. Thus, the gas is converted to a plasma. Electric energy is transmitted to the gas (to the plasma) and raises its inner energy. This inner energy is utilized when melting plastic powders so that the powder is fed to the out-streaming plasma (FIG. 2) wherein it is plasticized. The plasma spray accelerates the melt drops with a high rate on the surface of the piece to be coated.
The temperature of the plasma spray is very high; between about 7000.degree. C. and about 15000.degree. C. Due to this high temperature, the thermal radiation of the plasma is also very high. Some advantages are obtained from this radiation energy when melting plastic powders as the radiation energy increases the temperature of the working piece. This is advantageous with respect to the polymerization and thus, with respect to the formation of the coating.
A drawback of conventional plasma spraying is that the temperature of the plasma flame is too high with respect to the plastic, and the plastic tends to oxidize. Further disadvantages of conventional plasma spray are the low flowing rate of the gas and that the heat effect of the flame is too low to keep the compact pieces warm. Generally the plastics of Table 3 (below) are sprayed with conventional plasma in a conventional plasma spraying system. In other words, specialty plastics are not used as the disadvantages of using such specialty plastics have not been overcome.
TABLE 3 __________________________________________________________________________ A COMPARISON OF USUAL POWDERY COAT TYPES OF COATINGS THERMO SETTING PLASTICS Polyester Polyester THERMO PLASTICS Epoxy urethane TGIC Hybride Acryl Nylon PVC __________________________________________________________________________ Application/ 120-122 150-200 140-200 140-220 140-200 180-320 170-290 curing tempera- ture .degree.C. Thickness of &lt;1-12 &lt;1-3,0 &lt;1-4,0 &lt;1-4,0 &lt;1-3,0 4-12 10-20 the film (1) Hardness HB-5H HB-5H HB-5H H-2H 2H-5H Outer strength - + + - + + 0 Weather - + + - + + - strength QUV-strength + 0 0 - + 0 0 Solvent strength + 0 0 0 0 + - Chemical + + + + + + + strength Impact strength + + + + 0 + + __________________________________________________________________________ (1) Normal thickness range Much more thicker films can be used with some materials. The meanings of the symbols: + Generally preferable/acceptable 0 Sometimes preferable/acceptable - Generally not preferable/acceptable